Avoiding remembered-set maintenance overhead for memory segments known to be in a collection set

ABSTRACT

A garbage collector that employs the train algorithm to manage a generation in a computer system&#39;s dynamically allocated heap maintains for each of the generation&#39;s cars a respective remembered set that identifies all locations where references to objects in that car have been found by scanning locations identified by the mutator as having been modified. To avoid some of the expense of remembered-set updating, the collector refrains from attempting to add to a remembered set any reference located in a car that will be collected during the next collection increment. Additionally, if no mutator operation will occur before a collection set of one or more cars will be collected, any reference located outside that collection set but referring to an object within the collection set is not recorded in a remembered set but is recorded instead in a scratch-pad list of entries that identify references to collection-set objects that need to be evacuated.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention is directed to memory management. Itparticularly concerns what has come to be known as “garbage collection.”

[0003] 2. Background Information

[0004] In the field of computer systems, considerable effort has beenexpended on the task of allocating memory to data objects. For thepurposes of this discussion, the term object refers to a data structurerepresented in a computer system's memory. Other terms sometimes usedfor the same concept are record and structure. An object may beidentified by a reference, a relatively small amount of information thatcan be used to access the object. A reference can be represented as a“pointer” or a “machine address,” which may require, for instance, onlysixteen, thirty-two, or sixty-four bits of information, although thereare other ways to represent a reference.

[0005] In some systems, which are usually known as “object oriented,”objects may have associated methods, which are routines that can beinvoked by reference to the object. They also may belong to a class,which is an organizational entity that may contain method code or otherinformation shared by all objects belonging to that class. In thediscussion that follows, though, the term object will not be limited tosuch structures; it will additionally include structures with whichmethods and classes are not associated.

[0006] The invention to be described below is applicable to systems thatallocate memory to objects dynamically. Not all systems employ dynamicallocation. In some computer languages, source programs must be sowritten that all objects to which the program's variables refer arebound to storage locations at compile time. This storage-allocationapproach, sometimes referred to as “static allocation,” is the policytraditionally used by the Fortran programming language, for example.

[0007] Even for compilers that are thought of as allocating objects onlystatically, of course, there is often a certain level of abstraction tothis binding of objects to storage locations. Consider the typicalcomputer system 10 depicted in FIG. 1, for example. Data, andinstructions for operating on them, that a microprocessor 11 uses mayreside in on-board cache memory or be received from further cache memory12, possibly through the mediation of a cache controller 13. Thatcontroller 13 can in turn receive such data from system read/writememory (“RAM” 14 through a RAM controller 15 or from various peripheraldevices through a system bus 16. The memory space made available to anapplication program may be “virtual” in the sense that it may actuallybe considerably larger than RAM 14 provides. So the RAM contents will beswapped to and from a system disk 17.

[0008] Additionally, the actual physical operations performed to accesssome of the most-recently visited parts of the process's address spaceoften will actually be performed in the cache 12 or in a cache on boardmicroprocessor 11 rather than on the RAM 14, with which those cachesswap data and instructions just as RAM 14 and system disk 17 do witheach other.

[0009] A further level of abstraction results from the fact that anapplication will often be run as one of many processes operatingconcurrently with the support of an underlying operating system. As partof that system's memory management, the application's memory space maybe moved among different actual physical locations many times in orderto allow different processes to employ shared physical memory devices.That is, the location specified in the application's machine code mayactually result in different physical locations at different timesbecause the operating system adds different offsets to themachine-language-specified location.

[0010] Despite these expedients, the use of static memory allocation inwriting certain long-lived applications makes it difficult to restrictstorage requirements to the available memory space. Abiding by spacelimitations is easier when the platform provides for dynamic memoryallocation, i.e., when memory space to be allocated to a given object isdetermined only at run time.

[0011] Dynamic allocation has a number of advantages, among which isthat the run-time system is able to adapt allocation to run-timeconditions. For example, the programmer can specify that space should beallocated for a given object only in response to a particular run-timecondition. The C-language library function malloc( ) is often used forthis purpose. Conversely, the programmer can specify conditions underwhich memory previously allocated to a given object can be reclaimed forreuse. The C-language library function free( ) results in such memoryreclamation.

[0012] Because dynamic allocation provides for memory reuse, itfacilitates generation of large or long-lived applications, which overthe course of their lifetimes may employ objects whose total memoryrequirements would greatly exceed the available memory resources if theywere bound to memory locations statically.

[0013] Particularly for long-lived applications, though, allocation andreclamation of dynamic memory must be performed carefully. If theapplication fails to reclaim unused memory—or, worse, loses track of theaddress of a dynamically allocated segment of memory—its memoryrequirements will grow over time to exceed the system's availablememory. This kind of error is known as a “memory leak.”

[0014] Another kind of error occurs when an application reclaims memoryfor reuse even though it still maintains a reference to that memory. Ifthe reclaimed memory is reallocated for a different purpose, theapplication may inadvertently manipulate the same memory in multipleinconsistent ways. This kind of error is known as a “danglingreference,” because an application should not retain a reference to amemory location once that location is reclaimed. Explicit dynamic-memorymanagement by using interfaces like malloc( )/free( ) often leads tothese problems.

[0015] A way of reducing the likelihood of such leaks and related errorsis to provide memory-space reclamation in a more-automatic manner.Techniques used by systems that reclaim memory space automatically arecommonly referred to as “garbage collection.” Garbage collectors operateby reclaiming space that they no longer consider “reachable.” Staticallyallocated objects represented by a program's global variables arenormally considered reachable throughout a program's life. Such objectsare not ordinarily stored in the garbage collector's managed memoryspace, but they may contain references to dynamically allocated objectsthat are, and such objects are considered reachable. Clearly, an objectreferred to in the processor's call stack is reachable, as is an objectreferred to by register contents. And an object referred to by anyreachable object is also reachable.

[0016] The use of garbage collectors is advantageous because, whereas aprogrammer working on a particular sequence of code can perform his taskcreditably in most respects with only local knowledge of the applicationat any given time, memory allocation and reclamation require a globalknowledge of the program. Specifically, a programmer dealing with agiven sequence of code does tend to know whether some portion of memoryis still in use for that sequence of code, but it is considerably moredifficult for him to know what the rest of the application is doing withthat memory. By tracing references from some conservative notion of a“root set,” e.g., global variables, registers, and the call stack,automatic garbage collectors obtain global knowledge in a methodicalway. By using a garbage collector, the programmer is relieved of theneed to worry about the application's global state and can concentrateon local-state issues, which are more manageable. The result isapplications that are more robust, having no dangling references andfewer memory leaks.

[0017] Garbage-collection mechanisms can be implemented by various partsand levels of a computing system. One approach is simply to provide themas part of a batch compiler's output. Consider FIG. 2's simplebatch-compiler operation, for example. A computer system executes inaccordance with compiler object code and therefore acts as a compiler20. The compiler object code is typically stored on a medium such asFIG. 1's system disk 17 or some other machine-readable medium, and it isloaded into RAM 14 to configure the computer system to act as acompiler. In some cases, though, the compiler object code's persistentstorage may instead be provided in a server system remote from themachine that performs the compiling. The electrical signals that carrythe digital data by which the computer systems exchange that code areexamples of the kinds of electromagnetic signals by which the computerinstructions can be communicated. Others are radio waves, microwaves,and both visible and invisible light.

[0018] The input to the compiler is the application source code, and theend product of the compiler process is application object code. Thisobject code defines an application 21, which typically operates on inputsuch as mouse clicks, etc., to generate a display or some other type ofoutput. This object code implements the relationship that the programmerintends to specify by his application source code. In one approach togarbage collection, the compiler 20, without the programmer's explicitdirection, additionally generates code that automatically reclaimsunreachable memory space.

[0019] Even in this simple case, though, there is a sense in which theapplication does not itself provide the entire garbage collector.Specifically, the application will typically call upon the underlyingoperating system's memory-allocation functions. And the operating systemmay in turn take advantage of various hardware that lends itselfparticularly to use in garbage collection. So even a very simple systemmay disperse the garbage-collection mechanism over a number ofcomputer-system layers.

[0020] To get some sense of the variety of system components that can beused to implement garbage collection, consider FIG. 3's example of amore complex way in which various levels of source code can result inthe machine instructions that a processor executes. In the FIG. 3arrangement, the human applications programmer produces source code 22written in a high-level language. A compiler 23 typically converts thatcode into “class files.” These files include routines written ininstructions, called “byte codes” 24, for a “virtual machine” thatvarious processors can be software-configured to emulate. Thisconversion into byte codes is almost always separated in time from thosecodes' execution, so FIG. 3 divides the sequence into a “compile-timeenvironment” 25 separate from a “run-time environment” 26, in whichexecution occurs. One example of a high-level language for whichcompilers are available to produce such virtual-machine instructions isthe Java™ programming language. (Java is a trademark or registeredtrademark of Sun Microsystems, Inc., in the United States and othercountries.)

[0021] Most typically, the class files' byte-code routines are executedby a processor under control of a virtual-machine process 27. Thatprocess emulates a virtual machine from whose instruction set the bytecodes are drawn. As is true of the compiler 23, the virtual-machineprocess 27 may be specified by code stored on a local disk or some othermachine-readable medium from which it is read into FIG. 1's RAM 14 toconfigure the computer system to implement the garbage collector andotherwise act as a virtual machine. Again, though, that code'spersistent storage may instead be provided by a server system remotefrom the processor that implements the virtual machine, in which casethe code would be transmitted electrically or optically to thevirtual-machine-implementing processor.

[0022] In some implementations, much of the virtual machine's action inexecuting these byte codes is most like what those skilled in the artrefer to as “interpreting,” so FIG. 3 depicts the virtual machine asincluding an “interpreter” 28 for that purpose. In addition to orinstead of running an interpreter, many virtual-machine implementationsactually compile the byte codes concurrently with the resultant objectcode's execution, so FIG. 3 depicts the virtual machine as additionallyincluding a “just-in-time” compiler 29. We will refer to thejust-in-time compiler and the interpreter together as “executionengines” since they are the methods by which byte code can be executed.

[0023] Now, some of the functionality that source-language constructsspecify can be quite complicated, requiring many machine-languageinstructions for their implementation. One quite-common example is asource-language instruction that calls for 64-bit arithmetic on a 32-bitmachine. More germane to the present invention is the operation ofdynamically allocating space to a new object; the allocation of suchobjects must be mediated by the garbage collector.

[0024] In such situations, the compiler may produce “inline” code toaccomplish these operations. That is, all object-code instructions forcarrying out a given source-code-prescribed operation will be repeatedeach time the source code calls for the operation. But inlining runs therisk that “code bloat” will result if the operation is invoked at manysource-code locations.

[0025] The natural way of avoiding this result is instead to provide theoperation's implementation as a procedure, i.e., a single code sequencethat can be called from any location in the program. In the case ofcompilers, a collection of procedures for implementing many types ofsource-code-specified operations is called a runtime system for thelanguage. The execution engines and the runtime system of a virtualmachine are designed together so that the engines “know” whatruntime-system procedures are available in the virtual machine (and onthe target system if that system provides facilities that are directlyusable by an executing virtual-machine program.) So, for example, thejust-in-time compiler 29 may generate native code that includes calls tomemory-allocation procedures provided by the virtual machine's runtimesystem. These allocation routines may in turn invoke garbage-collectionroutines of the runtime system when there is not enough memory availableto satisfy an allocation. To represent this fact, FIG. 3 includes block30 to show that the compiler's output makes calls to the runtime systemas well as to the operating system 31, which consists of procedures thatare similarly system-resident but are not compiler-dependent.

[0026] Although the FIG. 3 arrangement is a popular one, it is by nomeans universal, and many further implementation types can be expected.Proposals have even been made to implement the virtual machine 27'sbehavior in a hardware processor, in which case the hardware itselfwould provide some or all of the garbage-collection function.

[0027] The arrangement of FIG. 3 differs from FIG. 2 in that thecompiler 23 for converting the human programmer's code does notcontribute to providing the garbage-collection function; that resultslargely from the virtual machine 27's operation. Those skilled in thatart will recognize that both of these organizations are merelyexemplary, and many modern systems employ hybrid mechanisms, whichpartake of the characteristics of traditional compilers and traditionalinterpreters both.

[0028] The invention to be described below is applicable independentlyof whether a batch compiler, a just-in-time compiler, an interpreter, orsome hybrid is employed to process source code. In the remainder of thisapplication, therefore, we will use the term compiler to refer to anysuch mechanism, even if it is what would more typically be called aninterpreter.

[0029] In short, garbage collectors can be implemented in a wide rangeof combinations of hardware and/or software. As is true of most of thegarbage-collection techniques described in the literature, the inventionto be described below is applicable to most such systems.

[0030] By implementing garbage collection, a computer system can greatlyreduce the occurrence of memory leaks and other software deficiencies inwhich human programming frequently results. But it can also havesignificant adverse performance effects if it is not implementedcarefully. To distinguish the part of the program that does “useful”work from that which does the garbage collection, the term mutator issometimes used in discussions of these effects; from the collector'spoint of view, what the mutator does is mutate active data structures'connectivity.

[0031] Some garbage-collection approaches rely heavily on interleavinggarbage-collection steps among mutator steps. In one type ofgarbage-collection approach, for instance, the mutator operation ofwriting a reference is followed immediately by garbage-collector stepsused to maintain a reference count in that object's header, and code forsubsequent new-object storage includes steps for finding space occupiedby objects whose reference count has fallen to zero. Obviously, such anapproach can slow mutator operation significantly.

[0032] Other approaches therefore interleave very fewgarbage-collector-related instructions into the main mutator process butinstead interrupt it from time to time to perform garbage-collectioncycles, in which the garbage collector finds unreachable objects andreclaims their memory space for reuse. Such an approach will be assumedin discussing FIG. 4's depiction of a simple garbage-collectionoperation. Within the memory space allocated to a given application is apart 40 managed by automatic garbage collection. In the followingdiscussion, this will be referred to as the “heap,” although in othercontexts that term refers to all dynamically allocated memory. Duringthe course of the application's execution, space is allocated forvarious objects 42, 44, 46, 48, and 50. Typically, the mutator allocatesspace within the heap by invoking the garbage collector, which at somelevel manages access to the heap. Basically, the mutator asks thegarbage collector for a pointer to a heap region where it can safelyplace the object's data. The garbage collector keeps track of the factthat the thus-allocated region is occupied. It will refrain fromallocating that region in response to any other request until itdetermines that the mutator no longer needs the region allocated to thatobject.

[0033] Garbage collectors vary as to which objects they considerreachable and unreachable. For the present discussion, though, an objectwill be considered “reachable” if it is referred to, as object 42 is, bya reference in the root set 52. The root set consists of referencevalues stored in the mutator's threads' call stacks, the CPU registers,and global variables outside the garbage-collected heap. An object isalso reachable if it is referred to, as object 46 is, by anotherreachable object (in this case, object 42). Objects that are notreachable can no longer affect the program, so it is safe to re-allocatethe memory spaces that they occupy.

[0034] A typical approach to garbage collection is therefore to identifyall reachable objects and reclaim any previously allocated memory thatthe reachable objects do not occupy. A typical garbage collector mayidentify reachable objects by tracing references from the root set 52.For the sake of simplicity, FIG. 4 depicts only one reference from theroot set 52 into the heap 40. (Those skilled in the art will recognizethat there are many ways to identify references, or at least datacontents that may be references.) The collector notes that the root setpoints to object 42, which is therefore reachable, and that reachableobject 42 points to object 46, which therefore is also reachable. Butthose reachable objects point to no other objects, so objects 44, 48,and 50 are all unreachable, and their memory space may be reclaimed.This may involve, say, placing that memory space in a list of freememory blocks.

[0035] To avoid excessive heap fragmentation, some garbage collectorsadditionally relocate reachable objects. FIG. 5 shows a typicalapproach. The heap is partitioned into two halves, hereafter called“semi-spaces.” For one garbage-collection cycle, all objects areallocated in one semi-space 54, leaving the other semi-space 56 free.When the garbage-collection cycle occurs, objects identified asreachable are “evacuated” to the other semi-space 56, so all ofsemi-space 54 is then considered free. Once the garbage-collection cyclehas occurred, all new objects are allocated in the lower semi-space 56until yet another garbage-collection cycle occurs, at which time thereachable objects are evacuated back to the upper semi-space 54.

[0036] Although this relocation requires the extra steps of copying thereachable objects and updating references to them, it tends to be quiteefficient, since most new objects quickly become unreachable, so most ofthe current semi-space is actually garbage. That is, only a relativelyfew, reachable objects need to be relocated, after which the entiresemi-space contains only garbage and can be pronounced free forreallocation.

[0037] Now, a collection cycle can involve following all referencechains from the basic root set—i.e., from inherently reachable locationssuch as the call stacks, class statics and other global variables, andregisters-and reclaiming all space occupied by objects not encounteredin the process. And the simplest way of performing such a cycle is tointerrupt the mutator to provide a collector interval in which theentire cycle is performed before the mutator resumes. For certain typesof applications, this approach to collection-cycle scheduling isacceptable and, in fact, highly efficient.

[0038] For many interactive and real-time applications, though, thisapproach is not acceptable. The delay in mutator operation that thecollection cycle's execution causes can be annoying to a user and canprevent a real-time application from responding to its environment withthe required speed. In some applications, choosing collection timesopportunistically can reduce this effect. Collection intervals can beinserted when an interactive mutator reaches a point at which it awaitsuser input, for instance.

[0039] So it may often be true that the garbage-collection operation'seffect on performance can depend less on the total collection time thanon when collections actually occur. But another factor that often iseven more determinative is the duration of any single collectioninterval, i.e., how long the mutator must remain quiescent at any onetime. In an interactive system, for instance, a user may never noticehundred-millisecond interuptions for garbage collection, whereas mostusers would find interruptions lasting for two seconds to be annoying.

[0040] The cycle may therefore be divided up among a plurality ofcollector intervals. When a collection cycle is divided up among aplurality of collection intervals, it is only after a number ofintervals that the collector will have followed all reference chains andbe able to identify as garbage any objects not thereby reached. Thisapproach is more complex than completing the cycle in a singlecollection interval; the mutator will usually modify references betweencollection intervals, so the collector must repeatedly update its viewof the reference graph in the midst of the collection cycle. To makesuch updates practical, the mutator must communicate with the collectorto let it know what reference changes are made between intervals.

[0041] An even more complex approach, which some systems use toeliminate discrete pauses or maximize resource-use efficiency, is toexecute the mutator and collector in concurrent execution threads. Mostsystems that use this approach use it for most but not all of thecollection cycle; the mutator is usually interrupted for a shortcollector interval, in which a part of the collector cycle takes placewithout mutation.

[0042] Independent of whether the collection cycle is performedconcurrently with mutator operation, is completed in a single interval,or extends over multiple intervals is the question of whether the cycleis complete, as has tacitly been assumed so far, or is instead“incremental.” In incremental collection, a collection cycle constitutesonly an increment of collection: the collector does not follow allreference chains from the basic root set completely. Instead, itconcentrates on only a portion, or collection set, of the heap.Specifically, it identifies every collection-set object referred to by areference chain that extends into the collection set from outside of it,and it reclaims the collection-set space not occupied by such objects,possibly after evacuating them from the collection set.

[0043] By thus culling objects referenced by reference chains that donot necessarily originate in the basic root set, the collector can bethought of as expanding the root set to include as roots some locationsthat may not be reachable. Although incremental collection therebyleaves “floating garbage,” it can result in relatively low pause timeseven if entire collection increments are completed during respectivesingle collection intervals.

[0044] Most collectors that employ incremental collection operate in“generations” although this is not necessary in principle. Differentportions, or generations, of the heap are subject to differentcollection policies. New objects are allocated in a “young” generation,and older objects are promoted from younger generations to older or more“mature” generations. Collecting the younger generations more frequentlythan the others yields greater efficiency because the youngergenerations tend to accumulate garbage faster; newly allocated objectstend to “die,” while older objects tend to “survive.”

[0045] But generational collection greatly increases what is effectivelythe root set for a given generation. Consider FIG. 6, which depicts aheap as organized into three generations 58, 60, and 62. Assume thatgeneration 60 is to be collected. The process for this individualgeneration may be more or less the same as that described in connectionwith FIGS. 4 and 5 for the entire heap, with one major exception. In thecase of a single generation, the root set must be considered to includenot only the call stack, registers, and global variables represented byset 52 but also objects in the other generations 58 and 62, whichthemselves may contain references to objects in generation 60. Sopointers must be traced not only from the basic root set 52 but alsofrom objects within the other generations.

[0046] One could perform this tracing by simply inspecting allreferences in all other generations at the beginning of every collectioninterval, and it turns out that this approach is actually feasible insome situations. But it takes too long in other situations, so workersin this field have employed a number of approaches to expeditingreference tracing. One approach is to include so-called write barriersin the mutator process. A write barrier is code added to a writeoperation to record information from which the collector can determinewhere references were written or may have been since the last collectioninterval. A reference list can then be maintained by taking such a listas it existed at the end of the previous collection interval andupdating it by inspecting only locations identified by the write barrieras possibly modified since the last collection interval.

[0047] One of the many write-barrier implementations commonly used byworkers in this art employs what has been referred to as the “cardtable.” FIG. 6 depicts the various generations as being divided intosmaller sections, known for this purpose as “cards.” Card tables 64, 66,and 68 associated with respective generations contain an entry for eachof their cards. When the mutator writes a reference in a card, it makesan appropriate entry in the card-table location associated with thatcard (or, say, with the card in which the object containing thereference begins). Most write-barrier implementations simply make aBoolean entry indicating that the write operation has been performed,although some may be more elaborate. The mutator having thus left arecord of where new or modified references may be, the collector canthereafter prepare appropriate summaries of that information, as will beexplained in due course. For the sake of concreteness, we will assumethat the summaries are maintained by steps that occur principally at thebeginning of each collection interval.

[0048] Of course, there are other write-barrier approaches, such assimply having the write barrier add to a list of addresses wherereferences where written. Also, although there is no reason in principleto favor any particular number of generations, and although FIG. 6 showsthree, most generational garbage collectors have only two generations,of which one is the young generation and the other is the maturegeneration. Moreover, although FIG. 6 shows the generations as being ofthe same size, a more-typical configuration is for the young generationto be considerably smaller. Finally, although we assumed for the sake ofsimplicity that collection during a given interval was limited to onlyone generation, a more-typical approach is actually to collect the wholeyoung generation at every interval but to collect the mature one lessfrequently.

[0049] Some collectors collect the entire young generation in everyinterval and may thereafter perform mature-generation collection in thesame interval. It may therefore take relatively little time to scan allyoung-generation objects remaining after young-generation collection tofind references into the mature generation. Even when such collectors douse card tables, therefore, they often do not use them for findingyoung-generation references that refer to mature-generation objects. Onthe other hand, laboriously scanning the entire mature generation forreferences to young-generation (or mature-generation) objects wouldordinarily take too long, so the collector uses the card table to limitthe amount of memory it searches for mature-generation references.

[0050] Now, although it typically takes very little time to collect theyoung generation, it may take more time than is acceptable within asingle garbage-collection interval to collect the entire maturegeneration. So some garbage collectors may collect the mature generationincrementally; that is, they may perform only a part of the maturegeneration's collection during any particular collection cycle.Incremental collection presents the problem that, since the generation'sunreachable objects outside the “collection set” of objects processedduring that cycle cannot be recognized as unreachable, collection-setobjects to which they refer tend not to be, either.

[0051] To reduce the adverse effect this would otherwise have oncollection efficiency, workers in this field have employed the “trainalgorithm,” which FIG. 7 depicts. A generation to be collectedincrementally is divided into sections, which for reasons about to bedescribed are referred to as “car sections.” Conventionally, ageneration's incremental collection occurs in fixed-size sections, and acar section's size is that of the generation portion to be collectedduring one cycle.

[0052] The discussion that follows will occasionally employ thenomenclature in the literature by using the term car instead of carsection. But the literature seems to use that term to refer variouslynot only to memory sections themselves but also to data structures thatthe train algorithm employs to manage them when they contain objects, aswell as to the more-abstract concept that the car section and managingdata structure represent in discussions of the algorithm. So thefollowing discussion will more frequently use the expression car sectionto emphasize the actual sections of memory space for whose managementthe car concept is employed.

[0053] According to the train algorithm, the car sections are groupedinto “trains,” which are ordered, conventionally according to age. Forexample, FIG. 7 shows an oldest train 73 consisting of a generation 74'sthree car sections described by associated data structures 75, 76, and78, while a second train 80 consists only of a single car section,represented by structure 82, and the youngest train 84 (referred to asthe “allocation train”) consists of car sections that data structures 86and 88 represent. As will be seen below, car sections' train membershipscan change, and any car section added to a train is typically added tothe end of a train.

[0054] Conventionally, the car collected in an increment is the oneadded earliest to the oldest train, which in this case is car 75. All ofthe generation's cars can thus be thought of as waiting for collectionin a single long line, in which cars are ordered in accordance with theorder of the trains to which they belong and, within trains, inaccordance with the order in which they were added to those trains.

[0055] As is usual, the way in which reachable objects are identified isto determine whether there are references to them in the root set or inany other object already determined to be reachable. In accordance withthe train algorithm, the collector additionally performs a test todetermine whether there are any references at all from outside theoldest train to objects within it. If there are not, then all carswithin the train can be reclaimed, even though not all of those cars arein the collection set. And the train algorithm so operates thatinter-car references tend to be grouped into trains, as will now beexplained.

[0056] To identify references into the car from outside of it,train-algorithm implementations typically employ “remembered sets.” Ascard tables are, remembered sets are used to keep track of references.Whereas a card-table entry contains information about references thatthe associated card contains, though, a remembered set associated with agiven region contains information about references into that region fromlocations outside of it. In the case of the train algorithm, rememberedsets are associated with car sections. Each remembered set, such as car75's remembered set 90, lists locations in the generation that containreferences into the associated car section.

[0057] The remembered sets for all of a generation's cars are typicallyupdated at the start of each collection interval. To illustrate how suchupdating and other collection operations may be carried out, FIG. 8depicts an operational sequence in a system of the typical typementioned above. That is, it shows a sequence of operations that mayoccur in a system in which the entire garbage-collected heap is dividedinto two generations, namely, a young generation and an old generation,and in which the young generation is much smaller than the oldgeneration. FIG. 8 is also based on the assumption and that the trainalgorithm is used only for collecting the old generation.

[0058] Block 102 represents a period of the mutator's operation. As wasexplained above, the mutator makes a card-table entry to identify anycard that it has “dirtied” by adding or modifying a reference that thecard contains. At some point, the mutator will be interrupted forcollector operation. Different implementations employ different eventsto trigger such an interruption, but we will assume for the sake ofconcreteness that the system's dynamic-allocation routine causes suchinterruptions when no room is left in the young generation for anyfurther allocation. A dashed line 103 represents the transition frommutator operation and collector operation.

[0059] In the system assumed for the FIG. 8 example, the collectorcollects the (entire) young generation each time such an interruptionoccurs. When the young generation's collection ends, the mutatoroperation usually resumes, without the collector's having collected anypart of the old generation. Once in a while, though, the collector alsocollects part of the old generation, and FIG. 8 is intended toillustrate such an occasion.

[0060] When the collector's interval first starts, it first processesthe card table, in an operation that block 104 represents. As wasmentioned above, the collector scans the “dirtied” cards for referencesinto the young generation. If a reference is found, that fact ismemorialized appropriately. If the reference refers to ayoung-generation object, for example, an expanded card table may be usedfor this purpose. For each card, such an expanded card table mightinclude a multi-byte array used to summarize the card's referencecontents. The summary may, for instance, be a list of offsets thatindicate the exact locations within the card of references toyoung-generation objects, or it may be a list of fine-granularity“sub-cards” within which references to young-generation objects may befound. If the reference refers to an old-generation object, thecollector often adds an entry to the remembered set associated with thecar containing that old-generation object. The entry identifies thereference's location, or at least a small region in which the referencecan be found. For reasons that will become apparent, though, thecollector will typically not bother to place in the remembered set thelocations of references from objects in car sections farther forward inthe collection queue than the referred-to object, i.e., from objects inolder trains or in cars added earlier to the same train.

[0061] The collector then collects the young generation, as block 105indicates. (Actually, young-generation collection may be interleavedwith the dirty-region scanning, but the drawing illustrates it forpurpose of explanation as being separate.) If a young-generation objectis referred to by a reference that card-table scanning has revealed,that object is considered to be potentially reachable, as is anyyoung-generation object referred to by a reference in the root set or inanother reachable young-generation object. The space occupied by anyyoung-generation object thus considered reachable is withheld fromreclamation. For example, it may be evacuated to a young-generationsemi-space that will be used for allocation during the next mutatorinterval. It may instead be promoted into the older generation, where itis placed into a car containing a reference to it or into a car in thelast train. Or some other technique may be used to keep the memory spaceit occupies off the system's free list. The collector then reclaims anyyoung-generation space occupied by any other objects, i.e., by anyyoung-generation objects not identified as transitively reachablethrough references located outside the young generation.

[0062] The collector then performs the train algorithm's central test,referred to above, of determining whether there are any references intothe oldest train from outside of it. As was mentioned above, the actualprocess of determining, for each object, whether it can be identified asunreachable is performed for only a single car section in any cycle. Inthe absence of features such as those provided by the train algorithm,this would present a problem, because garbage structures may be largerthan a car section. Objects in such structures would therefore(erroneously) appear reachable, since they are referred to from outsidethe car section under consideration. But the train algorithmadditionally keeps track of whether there are any references into agiven car from outside the train to which it belongs, and trains' sizesare not limited. As will be apparent presently, objects not found to beunreachable are relocated in such a way that garbage structures tend tobe gathered into respective trains into which, eventually, no referencesfrom outside the train point. If no references from outside the trainpoint to any objects inside the train, the train can be recognized ascontaining only garbage. This is the test that block 106 represents. Allcars in a train thus identified as containing only garbage can bereclaimed.

[0063] The question of whether old-generation references point into thetrain from outside of it is (conservatively) answered in the course ofupdating remembered sets; in the course of updating a car's rememberedset, it is a simple matter to flag the car as being referred to fromoutside the train. The step-106 test additionally involves determiningwhether any references from outside the old generation point into theoldest train. Various approaches to making this determination have beensuggested, including the conceptually simple approach of merelyfollowing all reference chains from the root set until those chains (1)terminate, (2) reach an old-generation object outside the oldest train,or (3) reach an object in the oldest train. In the two-generationexample, most of this work can be done readily by identifying referencesinto the collection set from live young-generation objects during theyoung-generation collection. If one or more such chains reach the oldesttrain, that train includes reachable objects. It may also includereachable objects if the remembered-set-update operation has found oneor more references into the oldest train from outside of it. Otherwise,that train contains only garbage, and the collector reclaims all of itscar sections for reuse, as block 107 indicates. The collector may thenreturn control to the mutator, which resumes execution, as FIG. 8B'sblock 108 indicates.

[0064] If the train contains reachable objects, on the other hand, thecollector turns to evacuating potentially reachable objects from thecollection set. The first operation, which block 110 represents, is toremove from the collection set any object that is reachable from theroot set by way of a reference chain that does not pass through the partof the old generation that is outside of the collection set. In theillustrated arrangement, in which there are only two generations, andthe young generation has previously been completely collected during thesame interval, this means evacuating from a collection set any objectthat (1) is directly referred to by a reference in the root set, (2) isdirectly referred to by a reference in the young generation (in which noremaining objects have been found unreachable), or (3) is referred to byany reference in an object thereby evacuated. All of the objects thusevacuated are placed in cars in the youngest train, which was newlycreated during the collection cycle. Certain of the mechanics involvedin the evacuation process are described in more detail in connectionwith similar evacuation performed, as blocks 112 and 114 indicate, inresponse to remembered-set entries.

[0065]FIG. 9 illustrates how the processing represented by block 114proceeds. The entries identify heap regions, and, as block 116indicates, the collector scans the thus-identified heap regions to findreferences to locations in the collectionset. As blocks 118 and 120indicate, that entry's processing continues until the collector finds nomore such references. Every time the collector does find such areference, it checks to determine whether, as a result of a previousentry's processing, the referred-to object has already been evacuated.If it has not, the collector evacuates the referred-to object to a(possibly new) car in the train containing the reference, as blocks 122and 124 indicate.

[0066] As FIG. 10 indicates, the evacuation operation includes more thanjust object relocation, which block 126 represents. Once the object hasbeen moved, the collector places a forwarding pointer in thecollection-set location from which it was evacuated, for a purpose thatwill become apparent presently. Block 128 represents that step.(Actually, there are some cases in which the evacuation is only a“logical” evacuation: the car containing the object is simply re-linkedto a different logical place in the collection sequence, but its addressdoes not change. In such cases, forwarding pointers are unnecessary.)Additionally, the reference in response to which the object wasevacuated is updated to point to the evacuated object's new location, asblock 130 indicates. And, as block 132 indicates, any referencecontained in the evacuated object is processed, in an operation thatFIGS. 11A and 11B (“FIG. 11”) depicts.

[0067] For each one of the evacuated object's references, the collectorchecks to see whether the location that it refers to is in thecollection set. As blocks 134 and 136 indicate, the reference processingcontinues until all references in the evacuated object have beenprocessed. In the meantime, if a reference refers to a collection-setlocation that contains an object not yet evacuated, the collectorevacuates the referred-to object to the train to which the evacuatedobject containing the reference was evacuated, as blocks 138 and 140indicate.

[0068] If the reference refers to a location in the collection set fromwhich the object has already been evacuated, then the collector uses theforwarding pointer left in that location to update the reference, asblock 142 indicates. The remembered set of the referred-to object's carwill have an entry that identifies the evacuated object's old locationas one containing a reference to the referred-to object. But theevacuation has placed the reference in a new location, for which theremembered set of the referred-to object's car may not have an entry.So, if that new location is not as far forward as the referred-toobject, the collector adds to that remembered set an entry identifyingthe reference's new region, as blocks 144 and 146 indicate. As thedrawing indicates, the remembered set may similarly need to be updatedeven if the referred-to object is not in the collection set.

[0069] Now, some train-algorithm implementations postpone processing ofthe references contained in evacuated collection-set objects until afterall directly reachable collection-set objects have been evacuated. Inthe implementation that FIG. 10 illustrates, though, the processing of agiven evacuated object's references occurs before the next object isevacuated. So FIGS. 11's blocks 134 and 148 indicate that the FIG. 11operation is completed when all of the references contained in theevacuated object have been processed. This completes FIG. 10'sobject-evacuation operation, which FIG. 9's block 124 represents.

[0070] As FIG. 9 indicates, each collection-set object referred to by areference in a remembered-set-entry-identified location is thusevacuated if it has not been already. If the object has already beenevacuated from the referred-to location, the reference to that locationis updated to point to the location to which the object has beenevacuated. If the remembered set associated with the car containing theevacuated object's new location does not include an entry for thereference's location, it is updated to do so if the car containing thereference is younger than the car containing the evacuated object. Block150 represents updating the reference and, if necessary, the rememberedset.

[0071] As FIG. 8's blocks 112 and 114 indicate, this processing ofcollection-set remembered set is performed initially only for entriesthat do not refer to locations in the oldest train. Those that do areprocessed only after all others have been, as blocks 152 and 154indicate.

[0072] When this process has been completed, the collection set's memoryspace can be reclaimed, as block 164 indicates, since no remainingobject is referred to from outside the collection set: any remainingcollection-set object is unreachable. The collector then relinquishescontrol to the mutator.

[0073] FIGS. 12A-12J illustrate results of using the train algorithm.FIG. 12A represents a generation in which objects have been allocated innine car sections. The oldest train has four cars, numbered 1.1 through1.4. Car 1.1 has two objects, A and B. There is a reference to object Bin the root set (which, as was explained above, includes live objects inthe other generations). Object A is referred to by object L, which is inthe third train's sole car section. In the generation's remembered sets170, a reference in object L has therefore been recorded against car1.1.

[0074] Processing always starts with the oldest train's earliest-addedcar, so the garbage collector refers to car 1.1's remembered set andfinds that there is a reference from object L into the car beingprocessed. It accordingly evacuates object A to the train that object Loccupies. The object being evacuated is often placed in one of theselected train's existing cars, but we will assume for present purposesthat there is not enough room. So the garbage collector evacuates objectA into a new car section and updates appropriate data structures toidentify it as the next car in the third train. FIG. 12B depicts theresult: a new car has been added to the third train, and object A isplaced in it.

[0075]FIG. 12B also shows that object B has been evacuated to a new caroutside the first train. This is because object B has an externalreference, which, like the reference to object A, is a reference fromoutside the first train, and one goal of the processing is to formtrains into which there are no further references. Note that, tomaintain a reference to the same object, object L's reference to objectA has had to be rewritten, and so have object B's reference to object Aand the inter-generational pointer to object B. In the illustratedexample, the garbage collector begins a new train for the car into whichobject B is evacuated, but this is not a necessary requirement of thetrain algorithm. That algorithm requires only that externally referencedobjects be evacuated to a newer train.

[0076] Since car 1.1 no longer contains live objects, it can bereclaimed, as FIG. 12B also indicates. Also note that the remembered setfor car 2.1 now includes the address of a reference in object A, whereasit did not before. As was stated before, remembered sets in theillustrated embodiment include only references from cars further back inthe order than the one with which the remembered set is associated. Thereason for this is that any other cars will already be reclaimed by thetime the car associated with that remembered set is processed, so thereis no reason to keep track of references from them.

[0077] The next step is to process the next car, the one whose index is1.2. Conventionally, this would not occur until some collection cycleafter the one during which car 1.1 is collected. For the sake ofsimplicity we will assume that the mutator has not changed anyreferences into the generation in the interim.

[0078]FIG. 12B depicts car 1.2 as containing only a single object,object C, and that car's remembered set contains the address of aninter-car reference from object F. The garbage collector follows thatreference to object C. Since this identifies object C as possiblyreachable, the garbage collector evacuates it from car set 1.2, which isto be reclaimed. Specifically, the garbage collector removes object C toa new car section, section 1.5, which is linked to the train to whichthe referring object F's car belongs. Of course, object F's referenceneeds to be updated to object C's new location. FIG. 12C depicts theevacuation's result.

[0079]FIG. 12C also indicates that car set 1.2 has been reclaimed, andcar 1.3 is next to be processed. The only address in car 1.3'sremembered set is that of a reference in object G. Inspection of thatreference reveals that it refers to object F. Object F may therefore bereachable, so it must be evacuated before car section 1.3 is reclaimed.On the other hand, there are no references to objects D and E, so theyare clearly garbage. FIG. 12D depicts the result of reclaiming car 1.3'sspace after evacuating possibly reachable object F.

[0080] In the state that FIG. 12D depicts, car 1.4 is next to beprocessed, and its remembered set contains the addresses of referencesin objects K and C. Inspection of object K's reference reveals that itrefers to object H, so object H must be evacuated. Inspection of theother remembered-set entry, the reference in object C, reveals that itrefers to object G, so that object is evacuated, too. As FIG. 12Eillustrates, object H must be added to the second train, to which itsreferring object K belongs. In this case there is room enough in car2.2, which its referring object K occupies, so evacuation of object Hdoes not require that object K's reference to object H be added to car2.2's remembered set. Object G is evacuated to a new car in the sametrain, since that train is where referring object C resides. And theaddress of the reference in object G to object C is added to car 1.5'sremembered set.

[0081]FIG. 12E shows that this processing has eliminated all referencesinto the first train, and it is an important part of the train algorithmto test for this condition. That is, even though there are referencesinto both of the train's cars, those cars' contents can be recognized asall garbage because there are no references into the train from outsideof it. So all of the first train's cars are reclaimed.

[0082] The collector accordingly processes car 2.1 during the nextcollection cycle, and that car's remembered set indicates that there aretwo references outside the car that refer to objects within it. Thosereferences are in object K, which is in the same train, and object A,which is not. Inspection of those references reveals that they refer toobjects I and J, which are evacuated.

[0083] The result, depicted in FIG. 12F, is that the remembered sets forthe cars in the second train reveal no inter-car references, and thereare no inter-generational references into it, either. That train's carsections therefore contain only garbage, and their memory space can bereclaimed.

[0084] So car 3.1 is processed next. Its sole object, object L, isreferred to inter-generationally as well as by a reference in the fourthtrain's object M. As FIG. 12G shows, object L is therefore evacuated tothe fourth train. And the address of the reference in object L to objectA is placed in the remembered set associated with car 3.2, in whichobject A resides.

[0085] The next car to be processed is car 3.2, whose remembered setincludes the addresses of references into it from objects B and L.Inspection of the reference from object B reveals that it refers toobject A, which must therefore be evacuated to the fifth train beforecar 3.2 can be reclaimed. Also, we assume that object A cannot fit incar section 5.1, so a new car 5.2 is added to that train, as FIG. 12Hshows, and object A is placed in its car section. All referred-toobjects in the third train having been evacuated, that (single-car)train can be reclaimed in its entirety.

[0086] A further observation needs to be made before we leave FIG. 12G.Car 3.2's remembered set additionally lists a reference in object L, sothe garbage collector inspects that reference and finds that it pointsto the location previously occupied by object A. This brings up afeature of copying-collection techniques such as the typicaltrain-algorithm implementation. When the garbage collector evacuates anobject from a car section, it marks the location as having beenevacuated and leaves the address of the object's new location. So, whenthe garbage collector traces the reference from object L, it finds thatobject A has been removed, and it accordingly copies the new locationinto object L as the new value of its reference to object A.

[0087] In the state that FIG. 12H illustrates, car 4.1 is the next to beprocessed. Inspection of the fourth train's remembered sets reveals nointer-train references into it, but the inter-generational scan(possibly performed with the aid of FIG. 6's card tables) revealsinter-generational references into car 4.2. So the fourth train cannotbe reclaimed yet. The garbage collector accordingly evacuates car 4.1'sreferred-to objects in the normal manner, with the result that FIG. 121depicts.

[0088] In that state, the next car to be processed has onlyinter-generational references into it. So, although its referred-toobjects must therefore be evacuated from the train, they cannot beplaced into trains that contain references to them. Conventionally, suchobjects are evacuated to a train at the end of the train sequence. Inthe illustrated implementation, a new train is formed for this purpose,so the result of car 4.2's processing is the state that FIG. 12Jdepicts.

[0089] Processing continues in this same fashion. Of course, subsequentcollection cycles will not in general proceed, as in the illustratedcycles, without any reference changes by the mutator and without anyaddition of further objects. But reflection reveals that the generalapproach just described still applies when such mutations occur.

[0090] Although automatic garbage collection tends to make programs morereliable, it can also slow their execution, so it is important to makethe garbage-collection operations as efficient as possible. Among theways of doing so is the above-mentioned use of remembered sets toexpedite finding references to collection-set objects. Remembered-setuse is beneficial not only in the train algorithm but also in othertechniques for incremental collection; other techniques, too, can dividememory into segments for which it maintains remembered sets. Butremembered-set maintenance itself imposes a cost. Adding aremembered-set entry is time-consuming because, among other things, itincludes testing for relative car location checking for duplicateentries.

SUMMARY OF THE INVENTION

[0091] I have found ways to reduce the expense of the updating operationin the train algorithm and other incremental-collection techniques. Oneway involves, before the updating operation occurs, having the collectoridentify at least part of what will become the next collectionincrement's collection set. Then, as the collector scans themutator-modified regions for references, it omits the attendantremembered-set updating in those cases in which the references therebyfound are located in the identified part of the collection set.

[0092] I have recognized that doing so does not compromise the operationof finding reachable objects. If the reference whose location is therebyomitted from a remembered set is not in a reference chain by which anobject in the collection set is reachable, no harm is done. If it is insuch a chain, on the other hand, it will be discovered during thecollection process without its having been recorded in a remembered set.This is because any reference in such a chain is located in an objectthat is reachable and that will therefore be evacuated. And, since thatobject is evacuated, it will be scanned for references to collection-setobjects. This will cause the omitted reference to be found.

[0093] Another way is applicable particularly to collectors that employthe train algorithm and use scratch-pad lists, associated withrespective trains, to list the locations where processing the collectionset's remembered-set entries revealed references to collection-setobjects. When the collector scans the mutator-modified regions inaccordance with this aspect of the invention, it sometimes also omitsthe attendant remembered-set updating in those cases in which thereferences thereby found refer to objects in the collection set.Instead, it places entries for those references directly intoscratch-pad lists. I have recognized that updating the remembered setand subsequently scanning the location thereby identified can be omittedin cases when no further reference modification will occur.

BRIEF DESCRIPTION OF THE DRAWINGS

[0094] The invention description below refers to the accompanyingdrawings, of which:

[0095]FIG. 1, discussed above, is a block diagram of a computer systemin which the present invention's teachings can be practiced;

[0096]FIG. 2 is, discussed above, is a block diagram that illustrates acompiler's basic functions;

[0097]FIG. 3, discussed above, is a block diagram that illustrates amore-complicated compiler/interpreter organization;

[0098]FIG. 4, discussed above, is a diagram that illustrates a basicgarbage-collection mechanism;

[0099]FIG. 5, discussed above, is a similar diagram illustrating thatgarbage-collection approach's relocation operation;

[0100]FIG. 6, discussed above, is a diagram that illustrates agarbage-collected heap's organization into generations;

[0101]FIG. 7, discussed above, is a diagram that illustrates ageneration organization employed for the train algorithm;

[0102]FIGS. 8A and 8B, discussed above, together constitute a flow chartthat illustrates a garbage-collection interval that includesold-generation collection;

[0103]FIG. 9, discussed above, is a flow chart that illustrates in moredetail the remembered-set processing included in FIG. 8A;

[0104]FIG. 10, discussed above, is a block diagram that illustrates inmore detail the referred-to-object evacuation that FIG. 9 includes;

[0105]FIGS. 11A and 11B, discussed above, together form a flow chartthat illustrates in more detail the FIG. 10 flow chart's step ofprocessing evacuated objects' references;

[0106] FIGS. 12A-12J, discussed above, are diagrams that illustrate acollection scenario that can result from using the train algorithm;

[0107]FIGS. 13A and 13B together constitute a flow chart thatillustrates a collection interval, as FIGS. 8A and 8B do, butillustrates optimizations that FIGS. 8A and 8B do not include;

[0108]FIGS. 14A and 14B together constitute a flow chart that describesan advantageous approach to memorializing the locations of newly writtenreferences;

[0109]FIG. 15 is a diagram that illustrates the addition of an entry toa remembered set; and

[0110]FIG. 16 is a flow chart that illustrates an improvement to theoperation, included in FIG. 11, of recording reference locations thathave changed because the objects containing them have been evacuated.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

[0111] As was indicated above, the present invention is not limited togarbage collectors that employ the train algorithm. And itsapplicability to train-algorithm-based collectors extends to collectorswhose operational sequences depart significantly from the sequence thatFIGS. 8-11 above illustrate. Although the sequence there illustratedcontemplates executing an entire collection increment in a singleinterval dedicated only to collection activity, there are ways ofspreading a collection increment over multiple intervals. Alternatively,most or all of the collection increment can be performed concurrentlywith mutator operation, although, as was indicated above, this tends tobe somewhat complex. Additionally, although the train algorithm isusually implemented in a multi-generational collector, there is noreason in principle why collectors that use the train algorithm need toemploy more than one generation.

[0112] In train-generation embodiments, it is necessary only that theevacuation policy tend to place related objects into common trains andthat the trains be checked for any strong references and reclaimed ifthey have none, as was explained above. Indeed, even in arrangements ofthe general type exemplified above in connection with FIGS. 8-11, inwhich an entire increment is performed in a single collection intervaldirected to a portion of an old generation, the sequence can differ fromthe one there illustrated. For example, FIGS. 13A and 13B (together,“FIG. 13”) depict in simplified form an approach that I prefer.

[0113] Whereas it was tacitly assumed above that, as is conventional,only a single car section would be collected in any given collectioninterval, the FIG. 13 sequence contemplates collecting more than asingle car during a collection increment. FIG. 13 also depicts certainoptimizations that some of the invention's embodiments may employ.Blocks 172, 176, and 178 represent operations that correspond to thosethat FIG. 8's blocks 102, 106, and 108 do, and dashed line 174represents the passage of control from the mutator to the collector, asFIG. 8's dashed line 104 does.

[0114] For the sake of efficiency, though, the collection operation ofFIG. 13 includes a step represented by block 180. In this step, thecollector reads the remembered set of each car in the collection set todetermine the location of each reference into the collection set from acar outside of it, it places the address of each reference thereby foundinto a scratch-pad list associated with the train that contains thatreference, and it places the scratch-pad lists in reverse-train order.As blocks 182 and 184 indicate, it then processes the entries in allscratch-pad lists but the one associated with the oldest train.

[0115] Before the collector processes references in that train'sscratch-pad list, the collector evacuates any objects referred to fromoutside the old generation, as block 186 indicates. To identify suchobjects, the collector scans the root set. In some generationalcollectors, it may also have to scan other generations for referencesinto the collection set. For the sake of example, though, we haveassumed the particularly common scheme in which a generation'scollection in a given interval is always preceded by complete collectionof every (in this case, only one) younger generation in the sameinterval. If, in addition, the collector's promotion policy is topromote all surviving younger-generation objects into older generations,it is necessary only to scan older generations, of which there are nonein the example; i.e., some embodiments may not require that the younggeneration be scanned in the block-186 operation.

[0116] For those that do, though, the scanning may actually involveinspecting each surviving object in the young generation, or thecollector may expedite the process by using card-table entries.Regardless of which approach it uses, the collector immediatelyevacuates into another train any collection-set object to which itthereby finds an external reference. The typical policy is to place theevacuated object into the youngest such train. As before, the collectordoes not attempt to evacuate an object that has already been evacuated,and, when it does evacuate an object to a train, it evacuates to thesame train each collection-set object to which a reference in thethus-evacuated object refers. In any case, the collector updates thereference to the evacuated object.

[0117] When the inter-generational references into the generation havethus been processed, the garbage collector determines whether there areany references into the oldest train from outside that train. If not,the entire train can be reclaimed, as blocks 188 and 190 indicate.

[0118] As block 192 indicates, the collector interval typically endswhen a train has thus been collected. If the oldest train cannot becollected in this manner, though, the collector proceeds to evacuate anycollection-set objects referred to by references whose locations theoldest train's scratch-pad list includes, as blocks 194 and 196indicate. It removes them to younger cars in the oldest train, againupdating references, avoiding duplicate evacuations, and evacuating anycollection-set objects to which the evacuated objects refer. When thisprocess has been completed, the collection set can be reclaimed, asblock 198 indicates, since no remaining object is referred to fromoutside the collection set: any remaining collection-set object isunreachable. The collector then relinquishes control to the mutator.

[0119]FIGS. 14A and 14B (together, “FIG. 14”) illustrate a way ofreducing the time required to perform FIG. 13's step 176, in which theresults of scanning “dirty” cards are recorded. As blocks 200 and 202indicate, any reference found in the dirty card is read to determinewhether it refers to a location in the young generation. If it does,that fact is memorialized appropriately, as block 204 indicates. Theparticular memorialization technique is not of concern here, but onepossible approach is to use an expanded card table. In such an expandedcard table, a multi-byte array would be used to summarize the card'sreference contents. The summary may be a list of offsets that indicatethe exact locations within the card where references to young-generationobjects may be found, for instance, or it may be a list offine-granularity “sub-cards” within which references to young-generationobjects may be found. In any event, the locations of suchinter-generational references must be memorialized in some fashion, andblock 204 represents such a memorialization step.

[0120] If the result of the block-202 test is that the referred-toobject is not in the young generation but rather is (in thistwo-generation example) in the old generation, then the reference'spresence must still be memorialized, too. As was discussed above, atypical way of so memorializing the presence of an (intra-generational)reference to an old-generation object is to install an appropriate entryinto a remembered set associated with the car section containing thereferred-to object.

[0121] Because of the need to avoid duplicate entries, installing aremembered-set entry can be time-consuming. To appreciate this, considerFIG. 15, which illustrates one way of adding an entry to a rememberedset's reference list. FIG. 15 depicts a memory space 206 allocated tothe remembered set's reference list as containing only sixteenreference-sized locations. Let us suppose that the reference of interestoccurs at a location whose address is 192E. To determine where to placethis address in the memory space 206 allocated to the reference list,the collector applies a hash function 208 to the address. In theillustrated example, the hash function is simply the address's fourleast-significant bits, whose hexadecimal representation is E_(H). Thecollector uses this value as an offset into the list, but it does notimmediately store the address at the list location thus identified. Itfirst reads that location's value to determine whether another addresshas already been stored there. In the FIG. 15 scenario, one already has,as its non-NULL contents indicate.

[0122] Now, if that already-stored address were itself 192E, resultingfrom an entry made during a previous collection interval, the collectorwould recognize that a duplicate had occurred, and it would stop itsattempt to store the value. But the already-stored address in theillustrated example is instead 477E, so the collector proceeds to thenext reference-list location. This location, too, contains a non-NULLvalue that differs from the address to be stored. Since that location isat the end of the list, the collector proceeds circularly to thebeginning of the list and repeats the test there. Again, the location isalready occupied, so it proceeds still further, and this time it findsan empty location.

[0123] Even though the collector has not inspected every list entry, itcan infer from encountering the empty location that the list has noduplicates of the entry to be added. Any previous attempt to store thesame value would have taken the same steps, and the collector wouldaccordingly have encountered the duplicate in attempting to find a spacefor this address. The collector therefore has simultaneously found alocation and avoided duplication.

[0124] Even with the atypically small list that FIG. 15 depicts, though,the task of avoiding duplication has been somewhat involved. Andremembered sets can become very large indeed, particularly if the carsection contains a “popular” object, one to which a large number ofother objects refer. In such a case, the task of adding a remembered-setentry can be quite costly. But I have recognized that, if the referenceis in fact in the next collection set, the time-consuming operation ofinstalling a remembered-set entry is unnecessary.

[0125] To understand why this is so, recall that the remembered set'spurpose is to keep track of references to objects so that the collectorcan ultimately determine whether those objects are reachable or,instead, are garbage. Also, consider a reference that is part of acollection-set object and refers to another old-generation object. Forsuch an object, there are two possibilities when the next old-generationcollection interval occurs, and neither necessitates recording thereference.

[0126] The first possibility is that the collection-set objectcontaining the reference in question is not referred to by any objectoutside the collection set (not even by any erst-while collection-setobject that has now been evacuated from the collection set because itwas potentially reachable). In that case, the object to which thereference in question refers is not reachable through that reference, sofailing to record that reference in a remembered set will not prevent areachable object from being identified as such.

[0127] The second possibility is that the collection-set objectcontaining the reference in question is indeed referred to by areference located outside of the collection set. In that case, theobject referred to by the reference in question is potentially reachablethrough that reference. But there is still no harm in not havingrecorded the subject reference's location in a remembered set as part ofthe card-scanning operation of FIG. 14. This is because, as wasexplained above in connection with FIG. 11's step 146, the necessaryremembered-set entry will be made as part of the operation of evacuatingthe object containing the reference. If the determination of FIG. 14'sstep 210 is that the reference is contained in the next collection set,therefore, there is no harm in refraining from recording the referenceand, as FIG. 14 indicates, simply determining whether any referencesremain.

[0128] If the reference is not located in the collection set, on theother hand, then its existence must be memorialized. Even in this case,though, some savings based on collection-set membership may be realized.To appreciate this, it helps to consider precisely what is meant by the“next” collection set to which block 210 refers, and reference to FIG.13 is helpful in this context. Recall that the collection intervalalways begins with the process of scanning dirty cards, as block 176indicates. As block 178 indicates, the young generation is thencollected. As was explained above, the collection interval endsimmediately after the young-generation collection in most cases; only anoccasional collection interval also includes some old-generationcollection, and the collection set is part of only the old generation.

[0129] During collection intervals that will not include old-generationcollection, the collector knows the identity of at least one car thatwill be in the collection set for the next collection interval thatincludes some old-generation collection. There may also be enough of theinput information used by its collection-set-selection routine that itcan identify more such cars. So FIG. 14's block-210 determination ofwhether the reference is in the “next” collection set can be made,although it may be based on only part of what the complete collectionset turns out to be.

[0130] But further savings can be realized when the reference-recordingoperation of FIG. 14 occurs during a collection interval that willinclude some old-generation collecting. As blocks 212 and 214 indicate,references not in the collection set will be entered in the appropriateremembered sets if the operation of FIG. 14 is being carried out duringa collection interval that will include only young-generationcollection, not old-generation collection. If the interval will includeold-generation collection, on the other hand, the references still haveto be recorded, but those that refer to objects in the next collectionset do not have to be recorded in the remembered sets.

[0131] To understand why, first recall that the entries in a rememberedsets associated with a given car identify locations at which referencesto objects in that car sections have been found, mostly during previouscollection intervals. During intervening mutator intervals, thosereferences may have disappeared. So old-generation collection includesagain scanning the locations identified by the remembered-set entries todetermine whether they still contain references to collection-setobjects. But, if FIG. 14's dirty-card scanning occurs during acollection interval in which a given car will be collected, anyreference found during the dirty-card scanning is guaranteed still to bepresent when that car section is collected. If that reference refers toa collection-set object, therefore, its location does not need to bescanned again. So it does not need to be placed in the car's rememberedset; as blocks 215 and 216 indicate, it can go directly to thescratch-pad list associated with the train to which the car containingthe reference belongs.

[0132] As block 200 indicates, the operation of scanning for referencescontinues until all references in the dirty section have been found.During most collection intervals, the procedure is finished when theyhave. In the particular intervals that will include some old-generationcollection, however, a mark can be made in an appropriate datastructure, as blocks 218 and 220 indicate, to indicate that the dirtycard has already been scanned for references to collection-set objects.That data structure can be consulted later, when the collection set'sremembered-set entries are being processed. If a collection-setremembered-set entry would otherwise cause the collector to scan alocation in that card, it refrains from doing so if the data structureindicates that the card has already been scanned.

[0133] So far, we have considered the “next” collection set to be eitherthe collection set that prevails for the current collection interval or,if the current collection interval includes no old-generationcollection, the collection set that will prevail during the nextcollection interval that does include some old-generation collection.But savings in remembered-set-entry installation can also be realizedduring an old-generation-collection interval by knowing the identitiesof cars contained in a subsequent collection interval's collection set.

[0134] Recall in this connection FIG. 11's processing of references thatevacuated objects contain. When an object is evacuated, the locations ofthe references that it contains are moved, too, so areference-containing object's removal must often be accompanied byadding remembered-set entries reflecting those references' newlocations, as FIG. 11's block 146 indicates. But the block-146 operationcan be so performed as to avoid such entry addition in some cases, asFIG. 16 illustrates.

[0135] As FIG. 16's blocks 222, 224, 226, and 228 indicate, the newlocation of a reference contained in an evacuated object is recorded inthe summary table if it refers to a young-generation object, and itnormally-but not always-is recorded in the appropriate remembered set ifit refers to an old-generation object. If it refers to an old-generationobject, the collector first determines, as block 228 indicates, whetherthe evacuated object containing it is now in the collection set thatwill prevail during the old-generation-collection cycle after thecurrent one. If it is, the collector refrains from recording itslocation in a remembered set; reasoning similar to that set forth inconnection with FIG. 14's block 210 establishes that in such a case noharm results from thus dispensing with remembered-set-entryinstallation.

[0136] The present invention thus reduces the cost of remembered-setmaintenance significantly and thus constitutes a significant advance inthe art.

What is claimed is:
 1. For employing a computer system that includes amemory to operate as a garbage collector that collects at least aportion of the memory in collection increments in which it collectsrespective collection sets of the memory and that treats at least aportion of the memory as divided into memory segments and maintainsremembered sets respectively associated with the memory segments, amethod comprising: A) during each collection increment: i) scanning forreferences to objects in the collection set the locations outside thecollection set identified by entries in each remembered set associatedwith a memory segment in the collection set; ii) evacuating from thecollection set any objects referred to by references thereby found; andiii) reclaiming the memory space occupied by the collection set; and B)before each of at least some collection increments: i) identifying acollection-set subset that includes at least a subset of the memorysegments that will belong to the collection set collected during thatcollection increment; and ii) performing reference-memorializationoperations in which: a) the locations of at least some references notlocated in the collection-set subset thus identified that refer toobjects are recorded in the remembered sets associated with the memorysegments containing those objects if those remembered sets do notalready include entries that identify them; and b) no reference that islocated in the collection-set subset thus identified is recorded in aremembered set.
 2. A method as defined in claim 1 wherein A) beforeobjects are evacuated during a given collection increment, at least onesaid collection-set subset for the next collection increment isidentified, B) some of the reference-memorialization operationsperformed before the next collection increment are performed in responseto the evacuation of a reference-containing object during the givencollection increment and include recording in remembered sets thelocations of the references contained by the reference-containingobjects, but no locations of references located in the collection-setsubset identified for the next collection set are thereby recorded.
 3. Amethod as defined in claim 1 wherein the collector collects at least aportion of the memory in accordance with the train algorithm, the memorysegments being car sections that the collector groups into trains.
 4. Amethod as defined in claim 3 wherein: A) during each collectionincrement, entries are placed into scratch-pad lists associated withrespective trains; B) those entries identify the locations of thereferences that were found by scanning the locations outside therespective collection set that were identified by entries in eachremembered set associated with a car section in the collection set; andC) in the reference-memorialization operation, the locations of at leastsome references to objects in the identified collection-set subset arerecorded in respective ones of the scratch-pad lists without havingfirst been recorded in the remembered sets associated with the carsections containing those objects.
 5. A method as defined in claim 4wherein: A) the garbage collector operates in collection intervals; B)in memorialization operations that occur during a collection interval inwhich a given collection set is reclaimed, the locations of referencesthat are located outside the given collection set, refer to objects inthe given collection set, and reside in locations identified as havingbeen modified since the last memorialization operation are recorded inrespective ones of the scratch-pad lists without having first beenrecorded in any of the remembered sets; and C) in memorializationoperations that occur during a collection interval in which a givencollection set is not reclaimed, the locations of references that arelocated outside the given collection set, refer to objects in the givencollection set, and reside in locations identified as having beenmodified since the last memorialization operation are recorded in theremembered sets associated with the car sections containing thoseobjects if those remembered sets do not already include entries thatidentify those references' locations.
 6. For employing a computer systemthat includes a memory to operate as a garbage collector that collectsat least a portion of the memory according to the train algorithm incollection increments in which it collects respective collection sets ofthe memory and that treats at least that portion of the memory asdivided into car sections grouped into trains and maintains rememberedsets respectively associated with the car sections, a method comprising:A) during each collection increment: i) providing scratch-pad listsassociated with respective trains; ii) scanning for references toobjects in the collection set the locations outside the collection setidentified by entries in each remembered set associated with a carsection in the collection set; iii) placing into the scratch-pad listsentries that identify the locations of the references thereby found; iv)evacuating from the collection set any objects referred to by referenceswhose locations the scratch-pad lists identify; and v) reclaiming thememory space occupied by the collection set; and B) before each of atleast some collection increments: i) identifying a collection-set subsetthat includes at least a subset of the car sections that will belong tothe collection set collected during that collection increment; and ii)performing reference-memorialization operations in which: a) thelocations of at least some references not located in the collection-setsubset thus identified that refer to objects are recorded in theremembered sets associated with the car sections containing thoseobjects if those remembered sets do not already include entries thatidentify them; and b) the locations of at least some references toobjects in the collection-set subset thus identified are, without havingfirst been recorded in the remembered sets associated with the carsections containing those objects, recorded in the scratch-pad listsassociated with the trains to which the car sections containing thosereferences belong.
 7. A method as defined in claim 6 wherein: A) thegarbage collector operates in collection intervals; B) inmemorialization operations that occur during a collection interval inwhich a given collection set is reclaimed, the locations of referencesthat are located outside the given collection set, refer to objects inthe given collection set, and reside in locations identified as havingbeen modified since the last memorialization operation are recorded inrespective ones of the scratch-pad lists without having first beenrecorded in any of the remembered sets; and C) in memorializationoperations that occur during a collection interval in which a givencollection set is not reclaimed, the locations of references that arelocated outside the given collection set, refer to objects in the givencollection set, and reside in locations identified as having beenmodified since the last memorialization operation are recorded in theremembered sets associated with the car sections containing thoseobjects if those remembered sets do not already include entries thatidentify those references' locations.
 8. A computer system comprising:A) processor circuitry operable to execute processor instructions; andB) memory circuitry, to which the processor circuitry is responsive,that contains processor instructions readable by the processor circuitryto configure the computer system as a garbage collector that: i)collects at least a portion of the memory in collection increments inwhich it collects respective collection sets of the memory; ii) treatsat least a portion of the memory as divided into memory segments; iii)maintains remembered sets respectively associated with the memorysegments; iv) during each collection increment: a) scans for referencesto objects in the collection set the locations outside the collectionset identified by entries in each remembered set associated with a carsection in the collection set; b) evacuates from the collection set anyobjects referred to by references thereby found; and c) reclaims thememory space occupied by the collection set; and v) before each of atleast some collection increments: a) identifies a collection-set subsetthat includes at least a subset of the car sections that will belong tothe collection set collected during that collection increment; and b)performs reference-memorialization operations in which: (1) thelocations of at least some references not located in the collection-setsubset thus identified that refer to objects are recorded in theremembered sets associated with the car sections containing thoseobjects if those remembered sets do not already include entries thatidentify them; and (2) no reference that is located in thecollection-set subset thus identified is recorded in a remembered set.9. A computer system as defined in claim 8 wherein: A) before objectsare evacuated during a given collection increment, at least one saidcollection-set subset for the next collection increment is identified,B) some of the reference-memorialization operations performed before thenext collection increment are performed in response to the evacuation ofa reference-containing object during the given collection increment andinclude recording in remembered sets the locations of the referencescontained by the reference-containing objects, but no locations ofreferences located in the collection-set subset identified for the nextcollection set are thereby recorded.
 10. A computer system as defined inclaim 8 wherein the collector collects at least a portion of the memoryin accordance with the train algorithm, the memory segments being carsections that the collector groups into trains.
 11. A computer system asdefined in claim 10 wherein: A) during each collection increment,entries are placed into scratch-pad lists associated with respectivetrains; B) those entries identify the locations of the references thatwere found by scanning the locations outside the respective collectionset that were identified by entries in each remembered set associatedwith a car section in the collection set; and C) in thereference-memorialization operation, the locations of at least somereferences to objects in the identified collection-set subset arerecorded in respective ones of the scratch-pad lists without havingfirst been recorded in the remembered sets associated with the carsections containing those objects.
 12. A computer system as defined inclaim 11 wherein: A) the garbage collector operates in collectionintervals; B) in memorialization operations that occur during acollection interval in which a given collection set is reclaimed, thelocations of references that are located outside the given collectionset, refer to objects in the given collection set, and reside inlocations identified as having been modified since the lastmemorialization operation are recorded in respective ones of thescratch-pad lists without having first been recorded in any of theremembered sets; and C) in memorialization operations that occur duringa collection interval in which a given collection set is not reclaimed,the locations of references that are located outside the givencollection set, refer to objects in the given collection set, and residein locations identified as having been modified since the lastmemorialization operation are recorded in the remembered sets associatedwith the car sections containing those objects if those remembered setsdo not already include entries that identify those references'locations.
 13. A computer system comprising: A) processor circuitryoperable to execute processor instructions; B) memory circuitry, towhich the processor circuitry is responsive, that contains processorinstructions readable by the processor circuitry to configure thecomputer system as a garbage collector that: i) collects at least aportion of the memory according to the train algorithm in collectionincrements in which it collects respective collection sets of thememory; ii) treats the at least a portion of the memory as divided intocar sections grouped into trains; iii) maintains remembered setsrespectively associated with the car sections; iv) during eachcollection increment: a) provides scratch-pad lists associated withrespective trains; b) scans for references to objects in the collectionset the locations outside the collection set identified by entries ineach remembered set associated with a car section in the collection set;c) places into the scratch-pad lists entries that identify the locationsof the references thereby found; d) evacuates from the collection setany objects referred to by references whose locations the scratch-padlists identify; and e) reclaims the memory space occupied by thecollection set; and v) before each of at least some collectionincrements: a) identifies a collection-set subset that includes at leasta subset of the car sections that will belong to the collection setcollected during that collection increment; and b) performsreference-memorialization operations in which: (1) the locations of atleast some references not located in the collection-set subset thusidentified that refer to objects are recorded in the remembered setsassociated with the car sections containing those objects if thoseremembered sets do not already include entries that identify them; and(2) the locations of at least some references to objects in thecollection-set subset thus identified are, without having first beenrecorded in the remembered sets associated with the car sectionscontaining those objects, recorded in the scratch-pad lists associatedwith the trains to which the car sections containing those referencesbelong.
 14. A computer system as defined in claim 13 wherein: A) thegarbage collector operates in collection intervals; B) inmemorialization operations that occur during a collection interval inwhich a given collection set is reclaimed, the locations of referencesthat are located outside the given collection set, refer to objects inthe given collection set, and reside in locations identified as havingbeen modified since the last memorialization operation are recorded inrespective ones of the scratch-pad lists without having first beenrecorded in any of the remembered sets; and C) in memorializationoperations that occur during a collection interval in which a givencollection set is not reclaimed, the locations of references that arelocated outside the given collection set, refer to objects in the givencollection set, and reside in locations identified as having beenmodified since the last memorialization operation are recorded in theremembered sets associated with the car sections containing thoseobjects if those remembered sets do not already include entries thatidentify those references' locations.
 15. A storage medium containinginstructions readable by a computer system that includes memory toconfigure the computer system to operate as a garbage collector that: A)collects at least a portion of the memory in collection increments inwhich it collects respective collection sets of the memory; B) treats atleast a portion of the memory as divided into memory segments; C)maintains remembered sets respectively associated with the memorysegments; D) during each collection increment: i) scans for referencesto objects in the collection set the locations outside the collectionset identified by entries in each remembered set associated with a carsection in the collection set; ii) evacuates from the collection set anyobjects referred to by references thereby found; and iii) reclaims thememory space occupied by the collection set; and E) before each of atleast some collection increments: i) identifies a collection-set subsetthat includes at least a subset of the car sections that will belong tothe collection set collected during that collection increment; and ii)performs reference-memorialization operations in which: a) the locationsof at least some references not located in the collection-set subsetthus identified that refer to objects are recorded in the rememberedsets associated with the car sections containing those objects if thoseremembered sets do not already include entries that identify them; andb) no reference that is located in the collection-set subset thusidentified is recorded in a remembered set.
 16. A storage medium asdefined in claim 15 wherein: A) before objects are evacuated during agiven collection increment, at least one said collection-set subset forthe next collection increment is identifled, B) some of thereference-memorialization operations performed before the nextcollection increment are performed in response to the evacuation of areference-containing object during the given collection increment andinclude recording in remembered sets the locations of the referencescontained by the reference-containing objects, but no locations ofreferences located in the collection-set subset identified for the nextcollection set are thereby recorded.
 17. A storage medium as defined inclaim 15 wherein the collector collects at least a portion of the memoryin accordance with the train algorithm, the memory segments being carsections that the collector groups into trains.
 18. A storage medium asdefined in claim 17 wherein: A) during each collection increment,entries are placed into scratch-pad lists associated with respectivetrains; B) those entries identify the locations of the references thatwere found by scanning the locations outside the respective collectionset that were identified by entries in each remembered set associatedwith a car section in the collection set; and C) in thereference-memorialization operation, the locations of at least somereferences to objects in the identified collection-set subset arerecorded in respective ones of the scratch-pad lists without havingfirst been recorded in the remembered sets associated with the carsections containing those objects.
 19. A storage medium as defined inclaim 18 wherein: A) the garbage collector operates in collectionintervals; B) in memorialization operations that occur during acollection interval in which a given collection set is reclaimed, thelocations of references that are located outside the given collectionset, refer to objects in the given collection set, and reside inlocations identified as having been modified since the lastmemorialization operation are recorded in respective ones of thescratch-pad lists without having first been recorded in any of theremembered sets; and C) in memorialization operations that occur duringa collection interval in which a given collection set is not reclaimed,the locations of references that are located outside the givencollection set, refer to objects in the given collection set, and residein locations identified as having been modified since the lastmemorialization operation are recorded in the remembered sets associatedwith the car sections containing those objects if those remembered setsdo not already include entries that identify those references'locations.
 20. A storage medium containing instructions readable by acomputer system that includes memory to configure the computer system tooperate as a garbage collector that: A) collects at least a portion ofthe memory according to the train algorithm in collection increments inwhich it collects respective collection sets of the memory; B) treatsthat portion of the memory as divided into car sections grouped intotrains; C) maintains remembered sets respectively associated with thecar sections; D) during each collection increment: i) providesscratch-pad lists associated with respective trains; ii) scans forreferences to objects in the collection set the locations outside thecollection set identified by entries in each remembered set associatedwith a car section in the collection set; iii) places into thescratch-pad lists entries that identify the locations of the referencesthereby found; iv) evacuates from the collection set any objectsreferred to by references whose locations the scratch-pad listsidentify; and v) reclaims the memory space occupied by the collectionset; and E) before each of at least some collection increments: i)identifies a collection-set subset that includes at least a subset ofthe car sections that will belong to the collection set collected duringthat collection increment; and ii) performs reference-memorializationoperations in which: a) the locations of at least some references notlocated in the collection-set subset thus identified that refer toobjects are recorded in the remembered sets associated with the carsections containing those objects if those remembered sets do notalready include entries that identify them; and b) the locations of atleast some references to objects in the collection-set subset thusidentified are, without having first been recorded in the rememberedsets associated with the car sections containing those objects, recordedin the scratch-pad lists associated with the trains to which the carsections containing those references belong.
 21. A storage medium asdefined in claim 20 wherein: A) the garbage collector operates incollection intervals; B) in memorialization operations that occur duringa collection interval in which a given collection set is reclaimed, thelocations of references that are located outside the given collectionset, refer to objects in the given collection set, and reside inlocations identified as having been modified since the lastmemorialization operation are recorded in respective ones of thescratch-pad lists without having first been recorded in any of theremembered sets; and C) in memorialization operations that occur duringa collection interval in which a given collection set is not reclaimed,the locations of references that are located outside the givencollection set, refer to objects in the given collection set, and residein locations identified as having been modified since the lastmemorialization operation are recorded in the remembered sets associatedwith the car sections containing those objects if those remembered setsdo not already include entries that identify those references'locations.
 22. An electromagnetic signal representing sequences ofinstructions that, when executed by a computer system that includesmemory, cause the computer system to operate as a garbage collectorthat: A) collects at least a portion of the memory in collectionincrements in which it collects respective collection sets of thememory; B) treats at least a portion of the memory as divided intomemory segments; C) maintains remembered sets respectively associatedwith the memory segments; D) during each collection increment: i) scansfor references to objects in the collection set the locations outsidethe collection set identified by entries in each remembered setassociated with a car in the collection set; ii) evacuates from thecollection set any objects referred to by references thereby found; andiii) reclaims the memory space occupied by the collection set; and E)before each of at least some collection increments: i) identifies acollection-set subset that includes at least a subset of the carsections that will belong to the collection set collected during thatcollection increment; and ii) performs reference-memorializationoperations in which: a) the locations of at least some references notlocated in the collection-set subset thus identified that refer toobjects are recorded in the remembered sets associated with the carsections containing those objects if those remembered sets do notalready include entries that identify them; and b) no reference that islocated in the collection-set subset thus identified is recorded in aremembered set.
 23. A electromagnetic signal as defined in claim 22wherein: A) before objects are evacuated during a given collectionincrement, at least one said collection-set subset for the nextcollection increment is identified, B) some of thereference-memorialization operations performed before the nextcollection increment are performed in response to the evacuation of areference-containing object during the given collection increment andinclude recording in remembered sets the locations of the referencescontained by the reference-containing objects, but no locations ofreferences located in the collection-set subset identified for the nextcollection set are thereby recorded.
 24. A electromagnetic signal asdefined in claim 22 wherein the collector collects at least a portion ofthe memory in accordance with the train algorithm, the memory segmentsbeing car sections that the collector groups into trains.
 25. Aelectromagnetic signal as defined in claim 24 wherein: A) during eachcollection increment, entries are placed into scratch-pad listsassociated with respective trains; B) those entries identify thelocations of the references that were found by scanning the locationsoutside the respective collection set that were identified by entries ineach remembered set associated with a car section in the collection set;and C) in the reference-memorialization operation, the locations of atleast some references to objects in the identified collection-set subsetare recorded in respective ones of the scratch-pad lists without havingfirst been recorded in the remembered sets associated with the carsections containing those objects.
 26. A electromagnetic signal asdefined in claim 25 wherein: A) the garbage collector operates incollection intervals; B) in memorialization operations that occur duringa collection interval in which a given collection set is reclaimed, thelocations of references that are located outside the given collectionset, refer to objects in the given collection set, and reside inlocations identified as having been modified since the lastmemorialization operation are recorded in respective ones of thescratch-pad lists without having first been recorded in any of theremembered sets; and C) in memorialization operations that occur duringa collection interval in which a given collection set is not reclaimed,the locations of references that are located outside the givencollection set, refer to objects in the given collection set, and residein locations identified as having been modified since the lastmemorialization operation are recorded in the remembered sets associatedwith the car sections containing those objects if those remembered setsdo not already include entries that identify those references'locations.
 27. An electromagnetic signal representing sequences ofinstructions that, when executed by a computer system that includesmemory, cause the computer system to operate as a garbage collectorthat: A) collects at least a portion of the memory according to thetrain algorithm in collection increments in which it collects respectivecollection sets of the memory; B) treats that portion of the memory asdivided into car sections grouped into trains; C) maintains rememberedsets respectively associated with the car sections; D) during eachcollection increment: i) provides scratch-pad lists associated withrespective trains; ii) scans for references to objects in the collectionset the locations outside the collection set identified by entries ineach remembered set associated with a car section in the collection set;iii) places into the scratch-pad lists entries that identify thelocations of the references thereby found; iv) evacuates from thecollection set any objects referred to by references whose locations thescratch-pad lists identify; and v) reclaims the memory space occupied bythe collection set; and E) before each of at least some collectionincrements: i) identifies a collection-set subset that includes at leasta subset of the car sections that will belong to the collection setcollected during that collection increment; and ii) performsreference-memorialization operations in which: a) the locations of atleast some references not located in the collection-set subset thusidentified that refer to objects are recorded in the remembered setsassociated with the car sections containing those objects if thoseremembered sets do not already include entries that identify them; andb) the locations of at least some references to objects in thecollection-set subset thus identified are, without having first beenrecorded in the remembered sets associated with the car sectionscontaining those objects, recorded in the scratch-pad lists associatedwith the trains to which the car sections containing those referencesbelong.
 28. A electromagnetic signal as defined in claim 27 wherein: A)the garbage collector operates in collection intervals; B) inmemorialization operations that occur during a collection interval inwhich a given collection set is reclaimed, the locations of referencesthat are located outside the given collection set, refer to objects inthe given collection set, and reside in locations identified as havingbeen modified since the last memorialization operation are recorded inrespective ones of the scratch-pad lists without having first beenrecorded in any of the remembered sets; and C) in memorializationoperations that occur during a collection interval in which a givencollection set is not reclaimed, the locations of references that arelocated outside the given collection set, refer to objects in the givencollection set, and reside in locations identified as having beenmodified since the last memorialization operation are recorded in theremembered sets associated with the car sections containing thoseobjects if those remembered sets do not already include entries thatidentify those references' locations.
 29. A garbage collectorcomprising: A) means for collecting at least a portion of a computersystem's memory in collection increments in which respective collectionsets of the memory are collected; B) means for treating at least aportion of the memory as divided into memory segments and maintainingremembered sets respectively associated with the memory segments; C)means for, during each collection increment: i) scanning for referencesto objects in the collection set the locations outside the collectionset identified by entries in each remembered set associated with a carsection in the collection set; ii) evacuating from the collection setany objects referred to by references thereby found; and iii) reclaimingthe memory space occupied by the collection set; and D) means for,before each of at least some collection increments: i) identifying acollection-set subset that includes at least a subset of the carsections that will belong to the collection set collected during thatcollection increment; and ii) performing reference-memorializationoperations in which: a) the locations of at least some references notlocated in the collection-set subset thus identified that refer toobjects are recorded in the remembered sets associated with the carsections containing those objects if those remembered sets do notalready include entries that identify them; and b) no reference that islocated in the collection-set subset thus identified is recorded in aremembered set.
 30. A garbage collector comprising: A) means forcollecting at least a portion of the memory according to the trainalgorithm in collection increments in which respective collection setsof the memory are collected; B) means for treating at least a portion ofthe memory as divided into car sections grouped into trains andmaintaining remembered sets respectively associated with the carsections; C) means for, during each collection increment: i) providingscratch-pad lists associated with respective trains; ii) scanning forreferences to objects in the collection set the locations outside thecollection set identified by entries in each remembered set associatedwith a car section in the collection set; iii) placing into thescratch-pad lists entries that identify the locations of the referencesthereby found; iv) evacuating from the collection set any objectsreferred to by references whose locations the scratch-pad listsidentify; and v) reclaiming the memory space occupied by the collectionset; and D) means for, before each of at least some collectionincrements: i) identifying a collection-set subset that includes atleast a subset of the car sections that will belong to the collectionset collected during that collection increment; and ii) performingreference-memorialization operations in which: a) the locations of atleast some references not located in the collection-set subset thusidentified that refer to objects are recorded in the remembered setsassociated with the car sections containing those objects if thoseremembered sets do not already include entries that identify them; andb) the locations of at least some references to objects in thecollection-set subset thus identified are, without having first beenrecorded in the remembered sets associated with the car sectionscontaining those objects, recorded in the scratch-pad lists associatedwith the trains to which the car sections containing those referencesbelong.